demo.yml

- name: Register host keys
  shell: "ssh-keyscan {{ hostvars[item]['ansible_ssh_host'] }}"
  with_flattened:
    - "{{ groups['elasticpress'] }}"
    - "{{ groups['logstash'] }}"
    - "{{ groups['webserver'] }}"
    - "{{ groups['mariadb'] }}"
    - "{{ groups['nfs-server'] }}"
  register: inventory_host_keys
  tags: [ 'monitmonit' ]

- name: Add to known_hosts
  known_hosts:
    host: "{{ hostvars[item.item]['ansible_ssh_host'] }}"
    key: "{{ item.stdout }}"
  with_items: inventory_host_keys.results
  when: "{{ item.stdout }} is defined"
  tags: [ 'monitmonit' ]